A large portion of the data storage in today's computers uses magnetic media typically in the form of a disk. Data is represented to a computer by a large number of bits (ones and zeroes) and stored on disks where each bit is represented by a transition, which results in a leakage magnetic field. In order to read or write the value of any given bit, a read/write transducer is used, which includes one portion for changing or writing to the disk and another portion for detecting or reading changes in the magnetic field from transitions.
In the write cycle, a large high frequency current is applied to the write coil, which magnetizes a yoke. The fringing field from a small gap in the yoke magnetizes the magnetic disk. The direction of the fringing field, and hence the magnetization of the disk switches when the current polarity is reversed. A transition is formed when the current polarity is reversed.
In the read portion, a magnetoresistive (MR) sensor that changes electrical resistance in response to a magnetic field is employed. Older sensors utilize the anisotropic magnetoresistive (AMR) effect where a read element resistance varies in proportion to the square of the cosine of the angle between the magnetization in the read element and the direction of a sense current flowing through the read element. Data is read by the sensor from magnetic transitions recorded in the media. The magnetic field, resulting from a transition, causes a change in the direction of the magnetization in the read element. The new magnetization direction changes the resistance of the read element with a corresponding change in the sense current or voltage.
The sensors currently used exhibit a form of magnetoresistance called giant magnetoresistance (GMR). The GMR effect occurs in multilayer thin films of alternating ferromagnetic and nonferromagnetic metals. A subset of the GMR device is the spin valve in which two ferromagnetic (FM) layers, a “free layer” and a “pinned layer”, are separated by a non-magnetic spacer layer. The resistance of a GMR film changes according to the cosine of the angle between the magnetization of the FM layers. Hence, when the magnetization directions in the two layers are parallel the resistance is at a minimum; when the magnetization directions are anti-parallel, the resistance is at a maximum. As the magnetic disk is rotating, leakage fields from the written transitions change the relative angle between the directions of magnetization of the pinned and free layers, which in turn, result in a change in electrical resistance of the sensor. For a fixed DC current through the sensor, this translates into a change in readback voltage. Thus, the readback voltage consists of a positive or negative voltage pulse for each written transition.
For maximum readback signal, the maximum DC current that does not degrade the sensor is required. Degradation occurs due to temperature rise resulting from Joule heating in the sensor. Higher currents imply higher temperatures, which translates into a decrease in lifetime reliability.
Further, this problem has become more severe as higher density magnetic media is required which requires smaller devices, which therefore have increased current density and greater heating problems. If the current flow is decreased, the devices are less sensitive and thus less able to detect the data on high-density magnetic media. The demand for higher reader sensitivity is also driven by weaker magnetic fields from the media with increasing areal density (capacity per unit area).
Solutions to these problems have been long sought, but have long eluded those skilled in the art.